This invention relates generally to nondestructive testing for structural defects, and more particularly the invention relates to the detection of hydrogen attack in steel products such as pipe through ultrasonic testing techniques.
Hydrogen damage or attack is produced in steels by a hydrogen reaction with carbides which forms methane gas and decarburizes the steel. This process lowers the fracture toughness of steel without necessarily reducing wall thickness. Detection of hydrogen attack is important to assure safe operation of boiler tubes, pressure vessels, and piping subject to such damage.
More particularly, hydrogen damage or attack is produced in steels exposed to a high-pressure hydrogen environment at high temperatures. Under such conditions, a chemical reaction occurs between hydrogen and carbides in steel to produce methane gas bubbles in the grain boundaries. As the bubbles grow, they interlink to form intergranular fissures or microcracks. The kinetics of hydrogen attack depend on several variables including the temperature, pressure, and fluid being contained. In general, the chemical reaction for hydrogen attack can be simplified to: EQU Fe.sub.3 C+2H.sub.2 .fwdarw.CH.sub.4 .fwdarw.3Fe (1)
In petrochemical plants, hydrogen for equation (1) is present in the fluid stream in a tube or pipe. In the case of boiler tubes in fossil plants, hydrogen can be generated by a corrosion reaction of iron with water: EQU 3Fe+4H.sub.2 O.fwdarw.Fe.sub.3 O.sub.4 +4H.sub.2 (2)
The hydrogen available from the reaction in equation (2) is then used to promote the reaction in equation (1). Because the reaction of equation 2 is on the inside surface face (ID), hydrogen damage in boiler tubes is usually associated with corrosion and pitting at the inside diameter. As the hydrogen interacts with the steel to form methane gas, the gas bubbles at the grain boundaries and the decarburization of the steel reduce the material's fracture toughness. This loss of structural integrity from hydrogen attack has been known to produce several failures in fossil fuel and petrochemical plants.
Heretofore, ultrasonic techniques have been employed to detect structural defects such as large isolated cracks and wall thinning due to corrosion. The total transit time of an ultrasonic wave transmitted to and reflected from the defect or inner wall is used to determine distances in locating flaws or the thickness of the wall. Such transit-time measurements are typically made in microseconds.
Theoretical and laboratory studies have indicated that microcracks caused by hydrogen attack will affect both attenuation and velocity of an acoustic wave in a steel body. However, use of conventional ultrasonic testing techniques to identify the location and magnitude of such microcracks has resulted in inconsistent results in the field applications.